1. Field of Invention
This invention relates to air bag restraining systems for motor vehicles and more particularly, to two-stage air bag restraining systems for motor vehicles.
2. Prior Art
Air bags are well known in the art as a means for easing impacts of a collision to motor vehicle passengers. The air bag, therefore, must be activated immediately at the moment of collision and a short inflating time is desirable. On the other hand, if the air bag inflates too rapidly, the passenger is injured by the inflating air bag. For example, assume that a child is standing on the passenger seat in front of the instrument panel. If the air bag is activated, the sudden and rapid inflation of the bag would throw the child off the seat and in all probability cause serious injury to the child. In order to prevent such injury by the inflating air bag, it is necessary that the inflating speed of the bag be reduced. Therefore, the two-stage air bag device has been proposed in order to satisfy both of these contradictory requirements as described above.
The two-stage air bag device works such that the air bag is inflated to a predetermined level during the initial stage of the collision and after a brief interval it is inflated completely. In prior art air bag devices, a predetermined amount of gas is generated in the initial stages of collision so as to inflate the air bag to some degree. A great quantity of gas is then generated after an interval established somewhere between several milliseconds to several tens of milliseconds. With such a two-stage air bag device, the passenger is lightly pressed into the seat and is able to maintain a safe position during the first stage expansion. It is the second stage or main expansive action which absorbs the impact completely.
The time interval between the first stage expansion and the second stage expansion is set in relation to the quantity of gas used in the first stage and other factors. However, an improper interval adversely affects the air bag as a safety device. For example, if the interval is too short, it will cause the injury by inflating air bag but if the interval is too long, the shock absorption action of the air bag will be weakened. For this reason, a high degree of precision is required for the delayed ignition signal generator whose function it is to activate the first pressure device and thereafter the second pressure device.
A typical conventional delayed ignition signal generator is made up of a delay circuit, etc. employing complex electronic circuitry. Because of the complexity of the system and the attended high cost, its applicability to such infrequently used devices such as air bags, is limited. A typical prior art device is equipped with at least two acceleration sensors wherein the first sensor activates a first pressure device upon detection of a collision and thereafter deceleration speed of the vehicle is measured by a second sensor which in turn activates a second pressure device. In addition to the need for two sensors, the prior art device has another drawback in that the effectiveness of the air bag device is adversely affected by the relatively imprecise sensors which cannot provide a stable delay time.